Motivation

U-Boot has only a reduced set of supported network protocols. A major gap is the lack of a TCP stack.

For booting a diskless computer this leaves us with BOOTP or DHCP to get the address of a boot script. TFTP can be used to load the boot script and the operating system kernel and initial file system (initrd).

These protocols are insecure. The client cannot validate the authenticity of the contacted servers. And the server cannot verify the identity of the client.

Furthermore the services providing the operating system loader or kernel are not the ones that the operation system will use. Especially in a SAN environment this makes updating the operating system a hassle. After installing a new kernel version the boot files have to be copied to the TFTP server directory.

The HTTPS protocol provides certificate based validation of servers. Sensitive data like passwords can be securely transmitted.

The iSCSI protocol is used for connecting storage attached networks. It provides mutual authentication using the CHAP protocol. It typically runs on a TCP transport.

Thus a better solution than DHCP/TFTP boot would be to load a boot script via HTTPS and to download any other files needed for booting via iSCSI.

An alternative to implementing these protocols in U-Boot is to use an existing software that can run on top of U-Boot. iPXE is the "swiss army knife" of network booting. It supports both HTTPS and iSCSI. It has a script engine for fine grained control of the boot process and can provide a command shell.

iPXE can be built as an EFI application (named snp.efi) which can be loaded and run by U-Boot.

Boot sequence

U-Boot loads the EFI application iPXE snp.efi using the bootefi command. This application has network access via the simple network protocol offered by U-Boot.

iPXE executes its internal script. This script may optionally chain load a secondary boot script via HTTPS. Or open a shell.

For the further boot process iPXE connects to the iSCSI server. This includes the mutual authentication using the CHAP protocol. After the authentication iPXE has access to the iSCSI targets.

For a selected iSCSI target iPXE sets up a handle with the block IO protocol. It uses the ConnectController boot service of U-Boot to request U-Boot to connect a file system driver. U-Boot reads from the iSCSI drive via the block IO protocol offered by iPXE. It creates the partition handles and install the simple file protocol. Now iPXE can call the simple file protocol to load Grub. U-Boot uses the block IO protocol offered by iPXE to fulfill the request.

Once Grub is started it uses the same simple file protocol to load Linux. Via the EFI stub Linux is called as an EFI application.

               +--------+          +--------+
               |        |          |        |
               |        | Runs     |        |
               | U-Boot |=========>| iPXE   |
               | EFI    |          | snp.efi|
+--------+     |        | DHCP     |        |
|        |<====|********|<=========|        |
| DHCP   |     |        | Request  |        |
| Server |     |        |          |        |
|        |====>|********|=========>|        |
+--------+     |        | Response |        |
               |        |          |        |
               |        |          |        |
+--------+     |        | HTTPS    |        |
|        |<====|********|<=========|        |
| HTTPS  |     |        | Request  |        |
| Server |     |        | Script   |        |
|        |====>|********|=========>|        |
+--------+     |        | Response |        |
               |        |          |        |
               |        |          |        |
+--------+     |        | iSCSI    |        |
|        |<====|********|<=========|        |
| iSCSI  |     |        | Auth     |        |
| Server |====>|********|=========>|        |
|        |     |        |          |        |
|        |     |        | Loads    |        |
|        |<====|********|<=========|        |        +--------+
|        |     |        | Grub     |        | Runs   |        |
|        |====>|********|=========>|        |=======>| Grub   |
|        |     |        |          |        |        |        |
|        |     |        |          |        |        |        |
|        |     |        |          |        | Loads  |        |
|        |<====|********|<=========|********|<=======|        |      +--------+
|        |     |        |          |        | Linux  |        | Runs |        |
|        |====>|********|=========>|********|=======>|        |=====>| Linux  |
|        |     |        |          |        |        |        |      |        |
+--------+     +--------+          +--------+        +--------+      |        |
                                                                     |        |
                                                                     |        |
                                                                     | ~ ~ ~ ~|

Security

The iSCSI protocol is not encrypted. The traffic could be secured using IPsec but iPXE does not support this. So we should at least separate the iSCSI traffic from all other network traffic. This can be achieved using a virtual local area network (VLAN).

                       +-----------+
                       |           |
                       |           |
                       |   iSCSI   |
                       |   Server  |
                       |           |
                       |           |
                       +-----------+
                             |
                             |iSCSI
                             |
+-----------+          +-----------+          +-----------+
|           |  VLAN 2  |     *     |          |           |
|           |----------|******     |          |           |
|  Diskless |          |  Managed  |          | Firewall  |
|  Computer |  VLAN 1  |  Switch   |   HTTP   |           |
|           |==========|***********|==========|***********|=====$
|           |          |           |          |           |
+-----------+          +-----------+          +-----------+

Configuration

iPXE

For running iPXE on arm64 the bin-arm64-efi/snp.efi build target is needed.

git clone http://git.ipxe.org/ipxe.git
cd ipxe/src
make bin-arm64-efi/snp.efi -j6 EMBED=myscript.ipxe
The script might look like this:

#!ipxe
echo iPXE script started
set user-class:hex 04:69:a0:45:43
set username user
set password password
dhcp
sanhook --no-describe iscsi:192.168.1.1:::1:iqn.2000-01.com.example:disk1 || goto fail
sanboot --no-describe --filename \EFI\debian\grubaa64.efi || goto fail
fail:
echo Opening shell
shell

The available commands for the boot script are documented at:

http://ipxe.org/cmd

Credentials are managed as environment variables. These are described here:

http://ipxe.org/cfg

iPXE by default will put the CPU to rest when waiting for input. U-Boot does not wake it up due to missing interrupt support. To avoid this behavior create file src/config/local/nap.h.

/* nap.h */
#undef NAP_EFIX86
#undef NAP_EFIARM
#define NAP_NULL

The supported commands in iPXE are controlled by an include, too. Putting the following into src/config/local/general.h is sufficient for most use cases.

/* general.h */
#define NSLOOKUP_CMD            /* Name resolution command */
#define PING_CMD                /* Ping command */
#define NTP_CMD                 /* NTP commands */
#define VLAN_CMD                /* VLAN commands */
#define IMAGE_EFI               /* EFI image support */
#define DOWNLOAD_PROTO_HTTPS    /* Secure Hypertext Transfer Protocol */
#define DOWNLOAD_PROTO_FTP      /* File Transfer Protocol */
#define DOWNLOAD_PROTO_NFS      /* Network File System Protocol */
#define DOWNLOAD_PROTO_FILE     /* Local file system access */

Notes

iPXE has recently changed the TPL level at which it is running. U-Boot v2018.03 requires the patches in https://patchwork.ozlabs.org/project/uboot/list/?series=35645.

Links